Controllable Synthesis of Concave Nanocubes, Right Bipyramids, and

Jun 11, 2013 - Zhibin Shao, Wei Zhu,* Hong Wang, Qianhui Yang, Shaolin Yang, Xiaodi Liu, and Guanzhong Wang*. Hefei National Laboratory for Physical ...
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Controllable Synthesis of Concave Nanocubes, Right Bipyramids, and 5‑Fold Twinned Nanorods of Palladium and Their Enhanced Electrocatalytic Performance Zhibin Shao, Wei Zhu,* Hong Wang, Qianhui Yang, Shaolin Yang, Xiaodi Liu, and Guanzhong Wang* Hefei National Laboratory for Physical Sciences at Microscale, and Department of Physics, University of Science and Technology of China, Hefei, Anhui, 230026, People’s Republic of China S Supporting Information *

ABSTRACT: Concave palladium nanocrystals are attractive for their superior catalytic ability arising from high densities of atomic steps and kinks. However, it is still a challenge to generate the concave surface, which is not favored by thermodynamics owing to its higher surface energy. In this study, concave palladium nanocubes have been synthesized kinetically in high yield via a facile one-step wet chemical method using sodium ascorbate (NaA) as the reductant in an aqueous solution. This process allows independent control of the average edge length and the surface curvature of the nanocubes, respectively. The particle morphology can be tuned by changing the reducing rate during the reaction. Right bipyramids and 5-fold twinned nanorods with concave surfaces have also been synthesized with two reductants at the different stages or an appropriate amount of ascorbic acid only. Remarkable enhancements in both electrocatalytic activity and stability are observed on concave Pd nanocubes and twinned nanocrystals over conventional Pd nanocrystals with flat surfaces and commercial Pd/C.

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INTRODUCTION

namically lowered the surface energy of concave surface and caused the formation of concave structure. They demonstrated that the angels of the concave structure of the cubic Pd nanocrystals could be roughly tuned by changing the CTAC/ CTAB ratio. Despite recent progress, it remains a challenge to fabricate Pd concave nanocrystals by a simple and rapid route in seed-free aqueous solutions and control the degree and morphology of concavities. In this work, we investigated Pd concave nanocubes synthesized from a fast one-step wet chemical method using NaA as the reductant in an aqueous solution. The surface curvature and morphology were modulated kinetically by the concentration of NaA in the initial stage of reaction and the reducing rate during the reaction, respectively. Concave right bipyramids and 5-fold twinned nanorods of Pd were also obtained by using L-ascorbic acid (AA) and NaA as reductants in turn or AA only at appropriate concentration. The asprepared Pd concave nanocrystals exhibited superior catalytic activity and stability in formic acid and ethanol electrooxidation over the normal Pd nanocrystals and commercial Pd/ C.

As a superior catalyst, palladium nanocrystals have gained exceptional attention in recent years.1−7 To optimize their catalytic ability, a variety of shapes have been synthesized and investigated, including cube, octahedron, tetrahedron, decahedron, icosahedron, bipyramid, plate, bar, rod, and wire.8−14 Corresponding research has demonstrated that the particle catalytic activity and stability are correlated with their shapes.15−18 Particularly, because of the presence of lowcoordinate atomic steps and kinks at high densities, nanoparticles with concave surfaces have intrinsically higher catalytic ability than their convex counterparts.19,20 Thus, the research of Pd nanocrystals with concave surface has been of particular interest. However, due to their higher surface energy, concave surfaces are much harder to form than convex ones on nanoparticles. Huang et al. reported the synthesis of Pd concave tetrahedral nanocrystals with {110} and {111} using a solvothermal method in the presence of formaldehyde.21 Also, various noble-metal concave nanocubes, including Pd,22 Pt,23 Rh,24 Ag,25 and Au−Pd,26 as well as Pd and Au concave twinned nanoparticles,27−29 have been prepared with seedmediated methods that boost directionally controlled overgrowth. In these works, curved structures were obtained through kinetically controlled synthesizing processes by tuning a set of reaction parameters like the concentration of precursor and reductant, the injection rate, and the reaction temperature. Zhang et al. reported the direct preparation of concave Pd nanocubes by the synergism of two capping reagents, cetyltrimethylammonium chloride (CTAC) and cetyltrimethylammonium bromide (CTAB).30 They concluded that the coadsorption of Br− and Cl− ions on the surface thermody© 2013 American Chemical Society

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MATERIALS AND METHODS Materials. Palladium nitrate dihydrate (PdNO3·2H2O, 39.5%), (+)-sodium L-ascorbate (NaA, 99%), L-ascorbic acid (AA, 99.7%), and cetyltrimethylammonium bromide (CTAB, Received: March 12, 2013 Revised: June 11, 2013 Published: June 11, 2013 14289

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99%) (all from Sinopharm Chemical Reagent Co., Ltd.) were introduced as purchased without further purification. Their aqueous solutions were freshly prepared before use. Synthesis of Concave Pd Nanocubes. In a typical synthesis of concave nanocubes, CTAB (182.0 mg) and NaA (19.8 mg) were dissolved in 15 mL of deionized water. The solution was put in a 50 °C water bath under magnetic stirring. When thermal equilibrium was established, 5 mL of Pd(NO3)2·2H2O (10.8 mg) aqueous solution was added in rapidly. In the reaction solution, the concentration of CTAB, NaA, and Pd(NO3)2·2H2O was 25 mM, 5 mM, and 0.8 mM, respectively. Under magnetic stirring, the combined solution was allowed to react for 5 min at 50 °C. Then, the solution was centrifuged, and the product was washed three times with water to remove excess CTAB before characterization. Synthesis of Concave Pd 5-fold Twinned Nanorods and Right Bipyramids. In a typical synthesis of concave 5fold twinned nanorods and right bipyramids, CTAB (182.0 mg) and AA (18.0 mg) were dissolved in 15 mL of deionized water. The solution was put in a 50 °C water bath under magnetic stirring. When thermal equilibrium was established, 5 mL of PdNO3·2H2O (10.8 mg) aqueous solution was added in rapidly. In the reaction solution, the concentration of CTAB, AA, and Pd(NO3)2·2H2O was 25 mM, 5 mM, and 0.8 mM, respectively. After 2 min, with magnetic stirring, 1 mL aqueous solution of NaA (19.8 mg) was added rapidly. In the reaction solution, the concentration of NaA was 5 mM. The final solution reacted for 5 min at 50 °C, still under magnetic stirring. Then, the solution was centrifuged, and the product was washed three times with water to remove excess CTAB before characterization. Characterization. Field-emission scanning electron microscope (FESEM; JEOL JSM-6700F) and high-resolution transmission electron microscope (HRTEM; JEOL model 2010) were used to characterize the morphology and structure of the concave Pd nanocrystals. X-ray diffraction (XRD; MAC MXPAHF) patterns and selected area electron diffraction (SAED) were used to determine their crystal structures. Electrochemical Measurements. The electrochemical measurements were performed at room temperature with a standard three-electrode system (CH Instrument, model 620B) with a platinum coil counter electrode and a saturated Ag/AgCl reference electrode. The working electrode was prepared by transferring 15 μL of the aqueous suspension of the prepared nanoparticles to a glassy carbon electrode with effective surface area of 0.196 cm2. After naturally drying in air for 2 h, the electrode was covered with 15 μL of Nafion dispersed in water (0.05%). With these electrodes, underpotential deposition of Cu was performed in a solution of 0.05 M H2SO4 and 0.05 M CuSO4 at 5 mV/s sweep rate. Formic acid electro-oxidation cyclic voltammetry (CV) measurements were conducted in a solution of 0.5 M H2SO4 and 0.5 M HCOOH at 50 mV/s sweep rate. Chronoamperometry (CA) experiments for catalyst stability were conducted at 0.3 VAg/AgCl bias.

Figure 1. SEM (a) and TEM (b) images of Pd concave nanocubes synthesized by reducing Pd(NO3)2·2H2O in an aqueous solution containing 5 mM NaA and 0.25 M CTAB. Models, SEM images, TEM images, electron diffraction patterns of concave nanocubes orientated along the [100] (c), [110] (d), and [111] (e) directions, respectively.

image (Figure 1b) present four pointed star shapes, confirming the concave structure of the surfaces. The universality of concave surfaces is further demonstrated in Figure 1, parts c, d, and e. X-ray diffraction was conducted on product nanocubes spread on a Si substrate. The pattern (Figure S1, all figures labeled by a leading S can be found in the Supporting Information, SI) with fcc Pd (JCPDS file no. 87-0641), while the exceptionally intense (200) peak suggests that most of the Pd nanocubes are preferentially oriented with their {100} facets parallel to the substrate. Both the selected area electron diffraction (SAED) and high resolution TEM measurements (Figure S2 of the SI ) on a single concave nanocube clearly indicate their single-crystalline nature. While the SAED pattern is indexed to the [001] zone axis of an fcc Pd single crystal, the HRTEM images show the fringes with lattice spacing of 0.20 nm, corresponding to the (200) lattice plane of fcc Pd. To understand the formation mechanism of Pd concave nanocubes, the reaction parameters were systematically investigated. With all other parameters constant with those in the typical process, Figure 2 shows that NaA concentration effectively modulates the degree of concavity of the nanocubes. The angle between the (100) facet and the concave surface gradually increases with the concentration of NaA from 2.5 to 7.5 mM, while the size of nanocubes slowly decreases. Homogeneous nucleation is maintained in the demonstrated NaA range. NaA is the key reductant due to its strong reducibility in our process that tunes the formation of concave Pd nanocubes. As discussed by Chernov et al. and Zhang et al.,31,32 when the side faces are capped by Br−, the reduced atoms prefer to nucleate and grow from the edges and corners. When the reducing rate of metal atoms is greater than the surface diffusion33 rate on the particle, the newly deposited

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RESULTS AND DISCUSSION Figure 1 shows SEM and TEM images of the product synthesized using the above-mentioned typical procedure for Pd nanocubes. The SEM image (Figure 1a) reveals that the product can be synthesized in high purity (>95%) and relatively large quantity. The as-prepared product has a cubic morphology with the edge length of 30 ± 3 nm. Each face of the cube is concave at the center. The nanocubes in the TEM 14290

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precursor concentration is deviated from optimized value, the shape of nanocubes tends to be irregular (Figure S5 of the SI). Since the reduction rate of Pd precursor is a key factor for the degree of cavity, it is interesting to investigate the effect of changing the reduction rate in the middle of the reaction. First, accelerating the reduction rate at an appropriate time after a low initial rate was attempted. In the procedure of synthesizing the Pd nanocubes with flat faces, 1 mL of 0.1 M NaA was added in at 50 s after the reaction start time. SEM and TEM images (Figure 3) of the product show that the edges and corners of

Figure 3. Schematic illustration of shape evolution, when accelerating the reduction rate at 50 s after a low initial rate was performed. (a) SEM images, (b) TEM images, and (c) HRTEM images of Pd nanocubes that were prepared with 2.5 mM NaA at 0 s and the additional 5 mM NaA at 50 s.

the nanocubes are more convex than those prepared by the standard procedure. This indicates that the sudden acceleration of the reduction rate results in preferred Pd deposition onto the edges and corners. A transformation from the conventional nanocubes to concave ones is achieved by accelerating the reducing rate. Second, accelerating or decelerating the reduction rate at an appropriate time after a high reducing rate at the beginning was attempted. In the procedure of synthesizing the concave nanocubes, 100 μL of 14 M HNO3 or 1 mL of 2 M NaOH was added into the reaction solution at t = 50 s. As the pH was decreased with the addition of HNO3, the reducing rate of Pd precursor decreased, and more Pd atoms adsorbed at edges and corners could migrate to side faces owing to surface diffusion. Figure 4, parts c and d, shows that the convex portion at edges and corners of cubes is more obtuse than that of the typical product. In contrast, NaOH increased pH and accelerated the reduction process. More adsorbed Pd atoms grew at edges and corners, leading to sharper cube edges than those under the typical and low pH conditions (Figure 4, parts a and b). Moreover, different from adding HNO3 at 50 s, if HNO3 is introduced at 30 s, then the concave structure on the product is either eliminated or replaced by some concave spots (Figure 4, parts e and f). This is because reducing the growth rate at the earlier reaction stage makes enough Pd atoms migrate to the {100} side faces and fill the cavity to form nearly flat faces. Like the case of adding HNO3, by adding a small amount of Pd(NO3)2 after the standard procedure, the product consists mainly of larger cubes (Figure S6 of the SI). It is believed that the weak reducibility from the residual of the reductant can no longer keep the great reducing rate, and the cavity can be filled by the Pd atoms

Figure 2. SEM images, TEM images, and HRTEM images of Pd nanocubes synthesized with different NaA concentration: (a−c) 2.5, (d−f) 3.3, (g−i) 4.2, (j−l) 5.0, and (m−o) 7.5 mM. The insets in SEM images are 300% enlarged. (p) Summary curve of the NaA concentration-dependent the angle between (100) and concave surface and average edge length of nanocubes.

atoms will not migrate timely to the side surfaces, causing the formation of concave nanocubes. However, when the NaA concentration exceeds 7.5 mM, the degree of concavity stops increasing. Instead, irregular shaped nanocubes with square convex on the surface start to appear (Figure S3 of the SI). Alternatively, the average size of the concave nanocubes is more sensitive to the concentration of CTAB (also a key factor in controlling the shape and size of Pd nanocrystals27). When CTAB concentrations are 50 mM and 12.5 mM, nanocubes obtained are ca. 50 and 20 nm in size, respectively (Figure S4 of the SI). However, the concentration of Pd(NO3)2·2H2O precursor is not a preferred tuning parameter. When the 14291

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preferential exposition of {111} planes with low surface energy, which causes the formation of twinned nanocrystals. A dose of NaA solution added in at 2 min after the reaction start time resulted concave product. Shown in the SEM and TEM images (Figure 5, parts e−h), it is identified that the product contains 24% of 5-fold twinned nanorods with starlike cross section, 42% of concave right bipyramids, and 34% of concave nanocubes. The nanocubes are analogous with those prepared with NaA as the reductant. Besides, many variation of triangular and diamond shaped concave nanoparticles are observed in the SEM and TEM images. Computer modeling (Figure S8 of the SI) demonstrates that they are the projections of concave right bipyramids. The high resolution TEM image (Figure S9 of the SI) of a right bipyramid viewed along [011] zone axes demonstrates the existence of {111} twin plane. The 5-fold twinned nanorods are exposed as either starlike cross section (the upper inset in Figure 5g) or star fruit like projections at different roll angles (Figure S10 of the SI). The electron diffraction pattern (Figure S11d of the SI) of a 5-fold nanorod corresponds to the superposition of square ⟨100⟩ and rectangular ⟨112⟩ zone patterns.34 The HRTEM image (Figure S11c of the SI) taken over a ridge at the five-pointed star end reveals the symmetric domains with a lattice spacing of 0.27 nm. Confirmed with the double diffraction patterns in the SAED image (Figure S11d of the SI), it is classified that the nanorods have a cyclic penta-tetrahedral twin structure. Just like the prior discussed growth mechanism for the concave single crystal nanocubes, the rapid reducing rate provided by supplemental NaA leads to preferred Pd atoms deposition on the edge zone of twinned seeds, which results concave twinned nanocrystals. Our investigation indicates that the formation of concave structure depends on the time of adding NaA. If NaA was introduced at 20 s (Figure S12a of the SI), then most of the products are nanocubes along with only a few pentagonal bipyramids. When the growth rate becomes too fast at the initiate stage, the compensation effect cannot be achieved anymore and twins are consumed, which leads to seeds evolving into single crystal nanomaterials like cubes. If NaA is introduced at a later time point, then the growth ends and few Pd atoms are left to reform the edges and corners. Another driving factor is the initial concentration of AA. The results in Figure S12b of the SI show that the formation of Pd concave twinned nanocrystals is maintained up to initial AA concentration of 15 mM. However, when AA concentration is increased to over 30 mM, 90% of the products are nanocubes (Figure S12c of the SI). Since the concave palladium nanocrystals have a high density of low-coordinate atomic steps and kinks, as well as a high surface-area-to-volume ratio, they are predicted to have enhanced catalytic performance. The influence on the catalytic activities was evaluated by electrocatalytic oxidation of formic acid. The electrochemically active surface area (ECSA) of Pd catalyst was calculated from the charges associated with the stripping of a Cu monolayer underpotentially deposited on their surface.35−37 As shown in Figure S13a of the SI, it is clear that the concave nanocrystals (1.3 cm2 for concave nanocubes and 1.1 cm2 for concave twinned-nanocrystals) have a higher surface area than those with flat facets (0.8 cm2 for nanocubes and 1.0 cm2 for twinned-nanocrystals). Figure 6a shows cyclic voltammograms (CVs) normalized against the ECSAs of glassy carbon electrodes modified with different Pd catalysts measured in a 0.5 M H2SO4 with 0.5 M HCOOH solution. The concave nanocubes exhibit a maximum current density of 8.5 mA cm−2,

Figure 4. Schematic illustration of shape evolution, when accelerating or decelerating the reduction rate at an appropriate time after a high reducing rate at the beginning was performed. SEM images and TEM images of Pd nanocubes prepared using the typical procedure, with additional (a, b) 1 mL of 2 M NaOH (t = 50 s), (c, d) 100 μL of 14 M HNO3 (t = 50 s), and (e, f) 100 μL of 14 M HNO3 (t = 30 s) added. The insets in SEM images are 300% enlarged.

migrating from the corners. The size fluctuation (see Figure S7 of the SI) of particles is coincident with the results caused by atoms surface diffusion. These outcomes indicate that the cavity morphology can be readily tuned by adjusting the reducing rate, and even over tuning of reductant or precursor concentration can lead concave nanocubes back to general nanocubes. Using AA as the reductant, as described in the second typical procedure, 5-fold twinned nanorods and right bipyramids were obtained as well as nanocubes (see Figure 5, parts a−d). As Xiong et al. reported, the reduction rate is critical to the formation of twinned nanostructures.10 The mild reducing power of AA ensures a slow reduction, the high surface energy which arises from twins forming can be compensated by the

Figure 5. SEM images (a), TEM images (b), high-magnification morphology images (c), and models (d) of 5-fold twinned nanorods, right bipyramids, and nanocubes of Pd, synthesized with 5 mM of AA as the reducing agent. And corresponding images (e−h) of concaved Pd nanocrystals, synthesized initially with AA, and accelerated by NaA at 120 s. 14292

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features. By controlling the concentration of CTAB and NaA, the size and the curvature of concave nanocubes can be individually controlled. By changing the reducing rate in the reaction, the product morphology can be designed. We have also presented a synthesis method of concave right bipyramids and 5-fold twinned nanorods. The electrochemical activity and stability of different shaped nanocrystals have been evaluated in electro-oxidation of formic acid and ethanol against commercial Pd/C. Compared with the general nanocrystals, concave nanocrystals perform the higher electrocatalytic ability that increases with the degree of concavity under our controlled range.

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ASSOCIATED CONTENT

S Supporting Information *

Additional figures (Figure S1−S16) about morphology, crystal structure, electrocatalytic performance of concave nanocrystals. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86-551-63603323; fax: +86-551-63606266; e-mail: [email protected] (W.Z.), [email protected] (G.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of China (Grant Nos. 50772110, 50721091), the National Basic Research Program of China (2009CB939901, 2011CB921400), and the Fundamental Research Funds for the Central Universities (Grant No. WK2030000004).

Figure 6. (a) Cyclic voltammograms (CV) of four different Pd nanocrystals and commercial Pd/C measured in 0.5 M H2SO4 + 0.5 M HCOOH solution at a scan rate of 50 mVs −1. (b) Corresponding chronoamperometry curves at 0.3 VAg/AgCl. The current values were normalized to ECSA.

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2.2 times higher than the conventional nanocubes (3.8 mA cm−2), and 9.4 times higher than the commercial Pd/C (0.9 mA cm−2). The concave twinned nanocrystals (11.0 mA cm−2) show the highest maximum current density, 1.8 times higher than the nanocrystals with flat surfaces (6.0 mA cm−2). Moreover, the mass activity of concave nanocrystals (see Figure S14a of the SI) is also higher than the conventional nanocrystals and commercial Pd/C. However, the concave nanocubes appear to have the same mass activity as that from the concave twinned-nanocrystals, responsible for the higher surface area. Chronoamperometry (CA) experiments at 0.3 VAg/AgCl in Figure 6b demonstrate that the electrochemical stability of concave nanocrystals is also superior to that of the conventional nanocrystals. Furthermore, the formic acid electro-oxidation activities and the electrochemical stabilities of five batches of concave nanocubes with comparable size but different degrees of concavity are investigated in Figure S15 of the SI. It shows that the more concavity results higher enhancement of catalytic ability. Similar behavior was also observed with ethanol electro-oxidation in alkaline solution (Figures S16 and S13b of the SI). The concave Pd nanocrystals demonstrate the best catalytic activity and stability.

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CONCLUSIONS In summary, we have presented a high-yielding, nonseededgrowth, one-step synthesis method of concave palladium nanocubes. By introducing a relatively strong reducing agent of NaA, the preferential overgrowth on both {110} and {111} facets can be achieved to yield Pd nanocubes with concave 14293

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