Article pubs.acs.org/cm
Shape-Controlled Synthesis of Palladium Single-Crystalline Nanoparticles: The Effect of HCl Oxidative Etching and FacetDependent Catalytic Properties Jinfeng Zhang, Chao Feng, Yida Deng,* Lei Liu, Yating Wu, Bin Shen, Cheng Zhong, and Wenbin Hu* State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, P. R. China S Supporting Information *
ABSTRACT: This paper reports a convenient and facile preparation of singlecrystalline palladium with controllable shape based on the reduction of kinetic control via hydrochloric acid oxidative etching. The concentration of HCl added to the reaction solution was found to be crucial for the shape evolution of palladium nanocrystals from nanocubes bounded by {100} facets to octahedrons by {111} facets. Palladium nanocubes can be readily obtained at a fast reduction rate without the involvement of additional HCl. With the introduction of a certain amount of HCl to the precursor solution, truncated nanocubes with {111} facets were formed, and the increase of HCl led to the slower reduction rate and the formation of palladium cuboctahedrons enclosed by six {100} facets and eight {111} facets at the expense of gradual shrinkage of {100} facets. The probable mechanism of morphological transformation was proposed upon a batch of experiments. The shape-dependent catalytic performances of as-obtained palladium nanocrystals were investigated by the structure-sensitive reaction of formic acid oxidation. It was found that catalytic activities of palladium nanocrystals displayed a strong dependence on the facets exposed on the surface, and cubic palladium exhibited the best catalytic performance compared with cubooctahedral and octahedral palladium nanocrystals.
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INTRODUCTION It was well-known that properties and promising applications in catalysis and electrocatalysis exhibited strong dependence on the shape and size of metal nanostructures.1−3 For instance, branched platinum and porous palladium nanostructures with high surface area, as well as palladium with concave high-index facets, demonstrated enhanced catalytic activities.4−6 Therefore, tremendous efforts have been devoted to the synthesis of metal nanocrystals with controllable shape in recent years.7−9 Palladium (Pd) has attracted considerable research interest due to its unique properties in many industrial applications. It has demonstrated exceptional performance in hydrogen storage and hydrogen sensing,10,11 as well as in Stille and Suzuki coupling reaction serving as an effective catalyst.12−14 Furthermore, it has exhibited excellent catalytic activity in electron oxidation of formic acid,15 CO oxidation,16 and the activation of molecular oxygen from inert ground triplet to highly reactive singlet for glucose oxidation.17 It also can catalyze organic hydrogenation reactions, such as unsaturated alcohols18,19 and alkene hydrogenation.20 On the other hand, it has been demonstrated that the catalytic activities of Pd nanocrystals are very sensitive to their sizes and nanostructures, especially the specific facets exposed on the surface. It has been verified that cubic Pd with {100} facets exhibits more favorable catalytic properties than octahedrons with {111} facets in CO and glucose oxidation reactions.17,21 Thus, it is of vital importance to acquire Pd nanocrystals with facet-dependent performance using a rational shape-controlled method, and many efforts have been devoted to the synthesis of © 2013 American Chemical Society
Pd nanostructures with cube, octahedron, rhombic dodecahedron, and other morphologies.22−24 Xu et al. synthesized different low-index Pd facets by a seed-mediated method with CTAB as surfactant and KI as additive.22 Likewise, Pd cuboctahedrons and octahedrons were prepared with halide additive by the complex two-step process using premade Pd nanocubes as seeds.4,23 Xia’s research group24 employed various reductants with different reducing power for shape-controlled synthesis of Pd nanocrystals in the water-phase system. Therein, citric acid and bromide ions were essential for the formation of {111} and {100} facets, respectively. The absence of bromide ions would lead to the formation of Pd cuboctahedrons bounded by both {111} and {100} facets. Despite the success of preparation for Pd nanostructures, it is still a challenge to realize the morphological transformation of Pd with a more simple and facile approach in the absence of premade seeds and extra additives, particularly the understanding of the mechanism of selective growth for special facets. Herein, we develop a facile strategy for the one-pot preparation to fabricate single-crystalline Pd with controllable shape, synthesize low-index Pd nanocrystals without the seedmediated method, or employ typical capping agents such as bromide or iodide ions in aqueous solution. There are two unique features of this strategy. First, it involves no premade seeds, and nucleation just takes place during the reaction. Second, based on this synthetic approach, the morphological Received: October 30, 2013 Published: December 31, 2013 1213
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TMB Measurements. An amount of 30 μL of 3,5,3′,5′tetramethylbenzidine solution (50 mM) and 3 mL of HAc/NaAc buffer solution (0.2 M:0.2 M) were mixed together. Then 50 μL of different Pd nanoparticle (0.5 mg/mL) aqueous suspension was added into the mixture solution at 10 °C. The samples were taken for UV− vis measurements at different times on a Lambda 750S UV−vis−NIR spectrophotometer.
transformation of Pd nanostructures can be directly achieved via varying the amounts of HCl. Strikingly, the shape evolution of Pd from nanocubes enclosed by {100} facets to octahedrons by {111} facets is largely dependent on HCl concentration in our studies. The reduction kinetics studies revealed that Pd nanocubes can transform into cuboctahedrons upon an increasing amount of HCl and finally to octahedrons with a further addition of the Pd precursor. The catalytic results showed that cubic Pd nanocatalysts with {100} facets are more catalytically active for formic acid and 3,5,3′,5′-tetramethylbenzidine oxidation than other Pd nanocrystals. It revealed that catalytic activities of palladium nanocrystals display a strong dependence on the facets exposed on the surface.
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RESULTS AND DISCUSSION In this study, it was found that the molar ratio of HCl to Pd precursor H2PdCl4 plays an important role in the typical synthesis of Pd nanocrystals. Thus, the resultant shape of Pd affected by varying amounts of HCl would be discussed first. With no addition of extra HCl to the reaction solution, that is, to make the molar ratio of HCl to Pd precursor be 0, the Pd nanocubes with average size of 17 nm were synthesized, and the transmission electron microscopy (TEM) images are shown in Figure 1a and b. Figure 1c shows the representative high-
EXPERIMENTAL SECTION
Chemicals. Palladium chloride (PdCl2, analytical grade), sodium chloride (NaCl, ≥99.5%), hydrochloric acid (HCl, analytical grade), cetylpyridinium chloride (CPC, C21H38ClN·H2O, ≥99.0%), and ascorbic acid (AA, C6H8O6, ≥99.7%) were used. All reagents were purchased from Sinopharm Chemical Reagent Co., Ltd. and were used as received. The deionized water used in all experiments with a resistivity of 18.2 MΩ·cm was prepared using an ultrapure water system (Millipore). Synthesis of Pd Nanocubes. In a typical synthesis, 0.0358 g of CPC was dissolved in 5 mL of H2O in a vial, followed by the addition of H2PdCl4 aqueous solution (10 mM, 1 mL) mixed with 1 mL of H2O. The mixture were heated at 80 °C with magnetic stirring to form a clear solution, then 500 μL of fresh prepared 20 mM AA was added slowly while stirring. The reaction solution turned dark brown rapidly, and then the mixture was cooled and left undisturbed for 1 h. The resulting products were separated via centrifugation and further purified with ethanol several times. Meanwhile, precipitates were collected and redispersed in a small amount of water. Synthesis of Pd Cuboctahedrons. The procedure was analogous to what was used for the Pd nanocubes, except that 1 mL of H2O was replaced by 1 mL of HCl (40 mM). Synthesis of Pd Octahedrons. Pd octahedrons were prepared under the synthetic conditions of Pd cuboctahedrons, except that the concentration of H2PdCl4 was increased from 10 to 20 mM. Characterizations. The morphology and structure of the products were recorded with a field-emission transmission electron microscope (JEOL JEM-2100F) operated at 200 kV. UV−vis absorption spectra for TMB measurements were performed on a Lambda 750S UV−vis− NIR spectrophotometer. The precise content of Pd catalysts in each sample was determined by inductively coupled plasma-mass spectroscopy (ICP-MS, 7500a). Electrochemical Measurements. Cyclic voltammograms (CVs) were performed in a conventional three-electrode cell using an electrochemical system (Parstat 2273). The cell consists of the glassy carbon electrode (GCE, 5 mm in diameter) serving as a working electrode, Ag/AgCl (3.5 M KCl) reference electrode, and Pt counter electrode. The GCE was carefully polished and washed before each experiment. The suspension with cubic, cubooctahedral, and octahedral Pd was dropped onto the surface of the GCE, respectively, with the same loading of 50 μg·cm−2. After the solution was dried at room temperature, 8.5 μL of Nafion solution (0.2 wt %) was added and dried before electrochemical measurements. Formic acid oxidation reaction measurements were carried out in a solution of 0.5 M H2SO4 containing 0.5 M HCOOH at a sweep rate of 50 mV·s−1. To measure the electrochemically active surface area (ECSA) of Pd nanocatalysts, cyclic voltammetry (CV) measurements were conducted in 0.5 M H2SO4 solution with a scan rate of 50 mV·s−1. The ECSA can be calculated by the equation: ECSA = Q/mq. Q was determined by the area of the oxygen desorption peak in the CV curve, and m was the loaded amount of Pd. Meanwhile, assume a value of 420 μC·cm−2 for q, which is the charge required for desorption of a monolayer of oxygen on the Pd surface.
Figure 1. TEM and HRTEM images as well as the corresponding SAED pattern (inset) of an individual Pd nanocrystal with cubic and cubooctahedral shape, respectively. The synthetic procedure was similar to the synthesis of cubic Pd of 17 nm in size, except for the addition of HCl at different concentrations. The molar ratio of HCl to H2PdCl4 was: (a−c) 0 for nanocubes and (d−f) 4 for cuboctahedrons.
resolution TEM (HRTEM) image of the cubic Pd nanocrystal and the corresponding selected area electron diffraction (SAED) pattern (inset). The continuous lattice fringes with an interplanar distance of 0.194 nm can be ascribed as {200} planes of face-centered cubic (fcc) Pd. As well as the inset image, both reveal that the cubic Pd is a single crystal enclosed by {100} facets. When different molar ratios of HCl to H2PdCl4 ranging from 2 to 8 were added into the solution, distinct nanostructures of Pd could be obtained. The images show that the main products take a cubic shape with slight truncation at the corners by introducing a moderately low concentration of 20 mM HCl which corresponds to the molar ratio of 2 (see Figures S1a and S1b in the Supporting Information). Upon a further increase in the molar ratio of HCl to H2PdCl4 for 4, cubooctahedral Pd nanostructures with more truncation of the corners can be achieved (Figure 1d and e), and the corresponding HRTEM image and SAED pattern (inset) are shown in Figure 1f. Evidently, both the HRTEM image and SAED pattern implicate the lattice spacing of 0.227 and 0.194 nm indexed as {111} and {200} facets of fcc Pd, respectively. Following this approach to achieving octahedral Pd nanocrystals with the increasing performance of {111} facets to {100} 1214
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facets, it is necessary to introduce more HCl to the synthetic system. However, it was found that this failed to produce the desired result, and truncated octahedral Pd along with obvious Pd multiply twinned structures formed when excessive 80 mM HCl with the molar ratio of 8 was added into the reaction solution (see Figure S1c and S1d, Supporting Information). It has been demonstrated that Pd truncated octahedrons can evolve into octahedrons with further addition of metal atoms into the {100} facets.25 Thus, more Pd precursor was provided for the formation of octahedrons based on the synthesis of cubooctahedral shape. As the concentration of H2PdCl4 was doubled, Pd {100} facets gradually disappeared, and cubooctahedral Pd finally grew into octahedrons bounded by eight triangular {111} facets. As illustrated in Figure 2a,
Figure 3. Schematic illustration of shape evolution of the Pd nanocrystal and (a−e) the corresponding TEM images for various morphologies, respectively (scale bars, 10 nm), where slight truncation at the corner of cubic Pd was induced by HCl oxidative etching in the early stage and then continuous atomic addition to {100} facets promotes the enlargement of {111} facets and finally results in the formation of octahedral Pd bounded by {111} facets.
Figure 2. (a) TEM image and (b) HRTEM image as well as the corresponding SAED pattern (inset) of an individual octahedral Pd nanocrystal. The synthetic procedure was the same as the synthesis of Pd cuboctahedrons, except that the molar ratio of 4 for HCl to H2PdCl4 doubled H2PdCl4 concentration to 20 mM.
octahedral Pd nanocrystals with a small amount of truncated octahedral shape were observed. The HRTEM image and SAED pattern (inset) of an individual Pd octahedron are shown in Figure 2b, and the 0.227 nm spacing of adjacent lattice fringes together with the SAED pattern indicate the formation of single-crystal octahedral Pd. On the basis of the experimental results above, the emergence and enlargement of {111} facets exhibit a strong dependence on the concentration of HCl for Pd nanocrystals. To understand the growth mechanism of different facets, it is crucial to decipher the role of HCl in this synthetic approach of Pd nanocrystals. It has been reported that an appropriate oxidative etchant can effectively diminish twinned structures and obtain high-yield single-crystalline nanoparticles.24 The use of oxidative etchants such as Fe3+ and O2/Cl− could oxidize and dissolve newly formed Pd atoms back to [PdCl4]2− ions for the preparation of palladium nanocrystals, achieving a controllable number of nuclei for size control.26,27 For the O2/Cl− pair, the oxidative etching function mainly working through oxygen could be effectively enhanced with the help of a corrosive chloride ion.2 On the basis of previous experimental results and the oxidative etching of HCl, a possible reaction mechanism was proposed, and the corresponding schematic was illustrated in Figure 3. In the typical synthesis of palladium nanocrystals with cetylpyridinium chloride serving as surfactant, the color of the mixture solution gradually turned dark brown after the injection of reductant ascorbic acid (AA) at 80 °C, indicating that Pd precursor was reduced to Pd atoms. In the case of the molar ratio of HCl to Pd precursor for 0, the solution turned brown relatively rapidly as soon as the reducing agent AA was introduced. The formation rate of Pd atoms was very fast, and instantaneous nucleation occurred. In this study, the surfactant
CPC is supposed to absorb to the surface and direct the growth of Pd nanostructures and finally promote the generation of single-crystal Pd nuclei with cubic shape. As the reaction proceeds, the Pd nanocube enclosed by {100} facets grows into larger size, as shown in Figure 3a. However, compared with the cubic nanostructure, the corners of the Pd nanocube were slightly truncated by introducing 20 mM HCl, and the final product was called a truncated nanocube, as illustrated in Figure 3b. This change is supposed to ascribe to the function of HCl oxidative etching. As reported,28 the surface charge would aggregate at the corners of cubic Pd because the electric field intensity had a tendency to gather at these sites when the solution was exposed to the illumination of light, compared with the solution completely placed in the dark. Therefore, with the involvement of an appropriate concentration of HCl, the oxidative etching would preferentially occur at the corners rather than on other sites. The etching induced by HCl could oxidize newly formed Pd atoms back to ions, competing with the reduction reaction of Pd atoms generated by the reduction of the Pd precursor with AA. As compared to other locations of cubic Pd, these two opposite processes slow down the addition rate of Pd atoms to the corners to a certain extent. As a result, the Pd {111} facets emerge gradually with some slight corner truncation of the cube as the reaction continues. When the molar ratio of HCl to H2PdCl4 was increased to 4, the portion of {100} facets was further lessened, whereas the {111} facets expanded for the Pd cuboctahedron (Figure 3c). A small amount of truncated octahedral Pd nanocrystals consisting of six square {100} and eight hexagonal {111} facets can also be observed in this synthesis, and the TEM image of a single particle was shown in 1215
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Figure 3d. The results indicate that the increase of HCl concentration would further enhance oxidative etching at the corner of cubic Pd, leading to the enlargement of {111} facets and the shrinkage of {100} facets with continuous atomic addition. The fast growing {100} facets shrinking step by step as the crystal growth continues eventually results in an increasing ratio of {111} to {100} facet areas and transforms cubes to cuboctahedrons in the product. It has been verified that octahedral Pd nanocrystals could not be obtained by simply further increasing the amount of HCl based on the synthesis of cuboctahedral Pd. The insufficient Pd precursor should be responsible for the failure of the formation of Pd octahedrons due to the lack of Pd atoms which can not supply enough and continuous atomic addition to {111} facets. Thus, more Pd precursors were added into the solution, and an individual octahedral Pd nanocrystal was shown in Figure 3e. Both thermodynamic and kinetic factors should be taken into consideration to explain the mechanism responsible for the formation of octahedral Pd nanocrystals. First, the Pd {100} facets are not thermodynamically favored because the surface energies are in the order of γ{111} < γ{100} < γ{110} for fcc structure. It is necessary to maximize the coverage of {111} facets, to obtain the smallest surface area and lower the total surface energy. Second, there is kinetic control for oxidative etching. On one hand, the etching function alters the growth rates of different facets at the early stage of the emergence of Pd {111}. Then the slow growing {111} facets enlarge gradually at the expense of shrinkage of Pd {100} facets. On the other hand, in the presence of sufficient Pd precursor, the addition of Pd atoms to the {100} facets is continuous and fast enough, and then the eventual disappearance of {100} facets becomes possible according to a growth model in which the final shape would be determined by the relative growth rate of different facets and faceted by the slow growing facets. Thus, upon an increase in H2PdCl4, the resultant products were thermodynamically favorable shape octahedrons enclosed by eight {111} facets along with disappearance of {100} facets. It is worth noting that the size of Pd nanocrystals achieved with further addition of HCl to Pd precursor solution was relatively larger than the nanocubes obtained without the involvement of extra HCl. As previously discussed, the introduction of enough HCl could induce etching, dissolve newly formed Pd atoms, and lower the reduction rate in the nucleation step. As the reduction rate becomes slow, the number of nuclei would substantially reduce. The higher the concentration of HCl, the slower the generation of Pd atoms, just as demonstrated by the change rate of the solution color from almost transparent to dark brown during the reaction process. Consequently, at the same concentration of the Pd precursor, fewer nuclei produce Pd particles with relatively large size. The results suggest that the addition of HCl could obtain a tight control over the reduction kinetics in the synthesis of Pd nanostructures. In addition, cubic Pd nanocrystals could also be obtained with a change of the concentration of AA to 40 mM based on the synthesis of cubooctahedral Pd nanocrystals, as shown in Figure 4a. Notably, the average size of Pd nanocubes was found to increase from 17 to 26 nm relative to those obtained in the typical procedure. Additionally, by further increasing AA to 100 mM, the resultant Pd nanostructures were almost the same as that in the presence of 40 mM AA (Figure 4b). Due to the reduction rate accelerated by more reductant, enough Pd atoms can supplement the removal of atoms from the corners of cubic
Figure 4. TEM images of Pd nanocrystals prepared under the same conditions as the synthesis of Pd cuboctahedrons, except for the variation of AA concentration. (a) 40 mM; (b) 100 mM.
Pd nuclei caused by etching. Therefore, the final products still take the cubic shape, rather than transform to Pd cuboctahedrons. This result is reasonable and can be regarded as further evidence of the generation of cubic Pd nuclei in the nucleation stage with a fast reduction rate. With the introduction of the chloride ion involving NaCl instead of HCl at the same concentration in the synthesis of Pd cuboctahedrons, the final products were truncated nanocubes rather than cuboctahedrons, as shown in Figure 5a and 5b. This
Figure 5. TEM images of Pd nanocrystals prepared under the same conditions as the synthesis of Pd cuboctahedrons, except that 40 mM HCl was replaced by the same concentration of (a, b) NaCl and (c, d) NaBr.
difference suggests that the etching ability of the O2/Cl− pair arising from HCl is higher than that with NaCl, which can be supported by the standard potentials29 O2 + 4H+ + 4e− → 2H 2O −
O2 + 2H 2O + 4e → 4OH
E = 1.229 V −
E = 0.401 V
(1) (2)
According to the formula above, the presence of an acid radical ion can greatly improve the oxidative strength of oxygen. In addition, based on the Nernst equation, the increasing concentration of the acid radical ion can substantially enhance the power of oxidative etching. As expected, the use of acid HCl and the increasing amount of this acid are beneficial to the 1216
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peak current for cubic Pd is 3 times higher than that of Pd cuboctahedrons and 6 times higher than that of Pd octahedrons. These results imply that the catalytic activities are sensitive to the morphologies of Pd nanocrystals, and cubic shapes with exposed {100} facets are the most favorable shape for formic acid oxidation. To further test the facet-dependent catalytic activities of three types of Pd nanocrystals, a 3,5,3′,5′-tetramethylbenzidine (TMB) oxidation reaction was employed to evaluate the capabilities of activating O2 dissolved in water. UV−vis spectra of these Pd nanostructures after mixing with TMB solution for 5 min were shown in Figure 7. The results show that TMB
improvement of oxidative etching, leading to larger expression of {111} facets. To understand the effect of different halides, replacing NaCl with the same concentration of NaBr, cubic Pd nanocrystals >26 nm in size formed (Figure 5c and 5d). As compared to the introduction of NaCl, bromide ions were found to retard the reduction of the Pd precursor based on a color change rate of the solution and facilitate the formation of {100} facets. This observation is consistent with the report that the presence of bromide ions can selectively chemisorb and strongly bond to {100} facets.30 The binding of bromide ions to the surface is so strong that the reduction rate was slowed down, leading to the decreasing number of nuclei and finally promoting the generation of Pd cubes with larger size than the ones in the typical synthesis. Xia and co-workers have demonstrated that the production of cubic Pd exhibited a strong dependence on bromide ions rather than on other ions such as chloride or iodide in ethylene glycol solution.30 However, in our approach, Pd cubes still can be synthesized by taking advantage of chloride instead of bromide ions, suggesting that the role of different halides is in connection with the surrounding environment including the use of surfactant. To satisfy the demands for a higher power density of the direct formic acid fuel cells (DFAFCs), it is of vital importance to discover a suitable catalyst.31 It was well-known that the facets exposed on the Pd nanocrystals have a strong influence on catalytic performance. To clarify the shape effect of Pd nanostructures in their catalytic properties, formic acid electrochemical oxidation was used for evaluating the catalytic activities of these Pd nanocatalysts in this study. To minimize the influence of size, Pd nanocubes of 26 nm in size instead of 17 nm were tested as catalysts, and cubooctahedral and octahedral Pd nanocatalysts were prepared as the methods mentioned above (see Table S1, Supporting Information). The electrochemically active surface areas (ECSAs) of these Pd nanocrystals were calculated according to the surface areas determined by the oxygen desorption cyclic voltammograms in 0.5 M H2SO4 solution (Figure S2, Supporting Information). Figure 6 shows cyclic voltammograms of Pd nanocubes, cuboctahedrons, and octahedrons for electro-oxidation of formic acid in a solution of 0.5 M H2SO4 containing 0.5 M HCOOH at room temperature, respectively. From Figure 6, it can be clearly seen that cubic Pd nanocatalysts exhibit the best catalytic activities among these three Pd nanostructures. The
Figure 7. UV−vis absorbance of varying Pd nanostructures for TMB oxidation measurements after mixing with TMB solution for 5 min.
oxidation can also occur with the addition of Pd nanocrystals in the absence of peroxide which is generally necessary for this oxidation reaction. Besides, cubic Pd nanocrystals enclosed by complete {100} facets exhibit optimal performance for the O2 activation process. Moreover, the results shown in Figure S3 (Supporting Information) demonstrated that it can induce the formation of the final yellow oxidation product of TMB for Pd nanocubes but still a green or yellow-green intermediate for cuboctahedrons and octahedrons in the case of sufficient reaction time. The trend of curves for TMB oxidation is found to be structure sensitive, and the superior catalytic activities on the nanocubes than that of cuboctahedrons and octahedrons may be attributed to the facet effect: the {100} facets have a stronger capability of generating active oxygen than the {111} facets.
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CONCLUSIONS In summary, a new approach for the preparation of palladium nanocrystals with low-index facets has been described. It has been demonstrated that the shape evolution of Pd nanostructures from nanocubes enclosed by {100} facets to octahedrons by {111} facets has a strong dependence on the concentration of HCl, which provides a simple means of adjusting the reduction kinetics. The increasing amount of HCl could induce more powerful oxidative etching preferentially occurring at the corners of cubic Pd nanocrystals, promote the formation and enlargement of Pd {111} facets at the cost of the shrinkage of {100} facets, and eventually result in the generation of thermodynamically favored shape with maximum expression of {111} facets. On the other hand, the involvement of a certain concentration of HCl could slow down the reduction rate, depress nucleation, and reduce the number of seeds in the
Figure 6. Cyclic voltammograms of varying Pd nanocrystals in 0.5 M H2SO4 + 0.5 M HCOOH solution for formic acid oxidation. The sweep rate was 50 mV·s−1. 1217
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(17) Long, R.; Mao, K.; Ye, X.; Yan, W.; Huang, Y.; Wang, J.; Fu, Y.; Wang, X.; Wu, X.; Xie, Y.; Xiong, Y. J. Am. Chem. Soc. 2013, 135, 3200−3207. (18) Bhattacharjee, S.; Dotzauer, D. M.; Bruening, M. L. J. Am. Chem. Soc. 2009, 131, 3601−3610. (19) Wilson, O. M.; Knecht, M. R.; Garcia-Martinez, J. C.; Crooks, R. M. J. Am. Chem. Soc. 2006, 128, 4510−4511. (20) Doyle, A. M.; Shaikhutdinov, S. K.; Freund, H. J. Angew. Chem., Int. Ed. 2005, 44, 629−631. (21) Jin, M.; Liu, H.; Zhang, H.; Xie, Z.; Liu, J.; Xia, Y. Nano Res. 2010, 4, 83−91. (22) Niu, W.; Zhang, L.; Xu, G. ACS Nano 2010, 4, 1987−1996. (23) Zhang, H.; Jin, M.; Wang, J.; Li, W.; Camargo, P. H. C.; Kim, M. J.; Yang, D.; Xie, Z.; Xia, Y. J. Am. Chem. Soc. 2011, 133, 6078−6089. (24) Wiley, B.; Herricks, T.; Sun, Y.; Xia, Y. Nano Lett. 2004, 4, 1733−1739. (25) Lim, B.; Jiang, M.; Tao, J.; Camargo, P. H. C.; Zhu, Y.; Xia, Y. Adv. Funct. Mater. 2009, 19, 189−200. (26) Xiong, Y.; Chen, J.; Wiley, B.; Xia, Y.; Yin, Y.; Li, Z. Nano Lett. 2005, 5, 1237−1242. (27) Xiong, Y.; Chen, J.; Wiley, B.; Xia, Y.; Shaul, A.; Yin, Y. J. Am. Chem. Soc. 2005, 127, 7332−7333. (28) (a) Xiong, Y.; Wiley, B.; Chen, J.; Li, Z. Y.; Yin, Y.; Xia, Y. Angew. Chem., Int. Ed. 2005, 44, 7913−7917. (b) Kottmann, J. P.; Martin, O. J. F.; Smith, D. R.; Schultz, S. Phys. Rev. B 2001, 64, 235402. (29) (a) Handbook of Chemistry and Physics, 60th ed.; Weast, R. C., Ed.; CRC Press: Boca Raton, FL, 1980. (b) Xiong, Y.; McLellan, J. M.; Chen, J.; Yin, Y.; Li, Z.; Xia, Y. J. Am. Chem. Soc. 2005, 127, 17118− 17127. (30) Xiong, Y.; Cai, H.; Wiley, B. J.; Wang, J.; Kim, M. J.; Xia, Y. J. Am. Chem. Soc. 2007, 129, 3665−3675. (31) (a) Zhou, W. P.; Lewera, A.; Larsen, R.; Masel, R. I.; Bagus, P. S.; Wieckowski, A. J. Phys. Chem. B 2006, 110, 13393−13398. (b) Zhou, W.; Lee, J. Y. J. Phys. Chem. C 2008, 112, 3789−3793.
nucleation stage, and subsequently Pd nanostructures with larger size are formed. The catalytic results of varying Pd nanocatalysts toward electrochemical oxidation of formic acid show that {100} facet-enclosed Pd nanocubes are more favorable than octahedrons with {111} facets for enhancing the catalytic performance, which indicates that the facets exposed on the surface have an important influence on the catalytic activities. It is expected that the approach in this work may provide a route to the synthesis and shape control of other metals, and further studies for designing efficient catalysts with more favored nanostructures will be continued.
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ASSOCIATED CONTENT
S Supporting Information *
Figures S1−S3 and Table S1 (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Science Fund for Distinguished Young Scholars (No. 51125016), the National Natural Science Foundation of China (No. 51001075, 51371119), and Program of Shanghai Subject Chief Scientist (No. 11XD1402700).
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