5-fold Twinned Nanowires and Single Twinned Right Bipyramids of Pd

We report that 5-fold twinned nanowires and single twinned right bipyramids of Pd with high yields can be selectively synthesized in a hydrophilic sys...
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5‑fold Twinned Nanowires and Single Twinned Right Bipyramids of Pd: Utilizing Small Organic Molecules To Tune the Etching Degree of O2/Halides Na Lu,† Wei Chen,*,† Guoyong Fang,† Bi Chen,† Keqin Yang,† Yun Yang,† Zhencai Wang,† Shaoming Huang,*,† and Yadong Li‡ †

Zhejiang Key Laboratory of Carbon Materials, College of Chemistry and Materials Engineering, Wenzhou University, Wenzhou, Zhejiang 325027, P. R. China ‡ Department of Chemistry, Tsinghua University, Beijing 100084, P. R. China S Supporting Information *

ABSTRACT: We report that 5-fold twinned nanowires and single twinned right bipyramids of Pd with high yields can be selectively synthesized in a hydrophilic system with the assistance of acetonitrile and ethanol, respectively. The controlled synthesis is based on an idea that small organic molecules (SOMs) that can attract halide ions via electrostatic interactions of different strengths could well adjust their activity to tune the etching degree of O2/halides for protecting the twinned Pd crystal nucleus. We consider that relatively stronger interaction between acetonitrile and halide ions for the formation of nanowires is due to the existence of three C−Hδ+ bonds induced by the electron-withdrawing CN group of CH3CN, which is confirmed by an as-called iodine starch test, 1H nuclear magnetic resonance spectra, and theoretical calculations. On the basis of this finding, we then have successfully expanded SOMs to other molecules, including acetone, 1,4dioxane, and 1,3,5-trioxane, which have a function similar to that of acetonitrile for the production of Pd nanowires, and 2propanol, which has a function similar to that of ethanol for the fabrication of right bipyramids. Although nanowires and bipyramids are both mainly bound by {001} planes, nanowires show better catalytic performance toward the reduction of 4nitrophenol, indicating that more twin boundaries could offer more active catalytic sites. This work not only provides new information to decrease the degree of etching of O2/halides for controlling twin structure of noble metals but also supports the idea that creating a twin structure is good for enhancing catalytic activity.

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INTRODUCTION Shape and microstructure are two of the most important issues that scientists often assess when employing noble metal nanocrystals as the basic materials in the fields of catalysis, optics, magnetics, and biochemistry,1−7 because they generally determine the performance of these metal materials.1−7 Therefore, controlled synthesis of noble metal nanocrystals has continued to attract the interest of researchers. In theoretical and experimental terms, noble metals often form various polyhedral shapes confined by low-index facets such as {111}, {100}, and {110} in different proportions. Many synthetic strategies have been reported to tailor exposed faces to control the final shape of the metal nanocrystals.8−18 Among them, most reports focused on Au and Ag. Comparatively, because of its intrinsic electronic structure and stronger interactions with oxygen, nucleation and growth of metal Pd seem to be more difficult to control, and relatively limited synthetic routes have been developed. 19−23 Using the preferential adsorption of halides on Pd{100} faces, Pd nanocubes, nanobars, 5-fold twinned nanowires (NWs), and © 2014 American Chemical Society

single twinned right bipyramids (structures enclosed by {001} facets with two equal right tetrahedra symmetrically connected by sharing the base) could be prepared.24−29 Strong adsorption of CO on Pd(111)30,31 or assistance from HCHO and oleylamine32 allowed well-controlled synthesis of Pd nanocrystals with the shape confined by (111) planes. Seedmediated growth is another useful method for tuning the shape of Pd nanocrystals.33−38 Nevertheless, strategies for the onestep controlled synthesis of differently shaped Pd nanocrystals with high purity in a hydrophilic system still need to be further developed. The microstructure is another vital element of a metal-based material that makes a significant contribution to its function.6,39−42 All known shaped Pd nanocrystals can be divided into two types. One is a single crystal and the other a twinned crystalline structure. Twinned structures of Pd often Received: December 26, 2013 Revised: February 20, 2014 Published: March 6, 2014 2453

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mL of water and 5.0 mL of CH3CN were displaced by 6.2 mL of water and 6.0 mL of ethanol, Pd right bipyramids were obtained after reaction for 8 h. Evaluation of Catalytic Activity. For catalytic reduction of 4-NP, aqueous solutions of 4-NP (0.036 mL, 0.01 M), NaBH4 (0.24 mL, 0.5 M), and a proper amount of water were added to a quartz cuvette while its contents were being stirred. Finally, the metallic catalyst (∼7.1 μg, based on the ICP-MS results) was quickly injected into the reaction system. The total volume of the solution was kept as 3 mL. As the reaction progressed, the bright yellow color of the solution gradually faded. The intensity of the absorption at 400 nm of the solution was recorded during the course of the reactions. The reaction equation is given in the Supporting Information. Characterization. Field emission scanning electron microscopy (FESEM) images were recorded on a Nova NanoSEM 200 scanning electron microscope. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images and EDX spectra were recorded on a JEOL JEM-1200EX transmission electron microscope and FEI Tecnai G2 F20 S-Twin instrument working at 300 kV. The concentration of Pd was measured by inductively coupled plasma mass spectrometry (ICP-MS). A UV2501PC (Shimadzu) spectrometer was used to measure the UV−vis absorption intensity of the solution to monitor the real-time variation of the concentration of 4-NP.

contain pentagonal twinned and single twinned configurations. Notably, the strain and/or defects in twinned area could result in better catalytic activity. For example, Yang’s group has demonstrated that the icosahedral Pt and Pt-based nanocrystals possessed activity toward the oxygen reduction reaction that was greater than that of single-crystalline octahedra.6,40 Nevertheless, during synthesis, the twin boundaries of the crystal nucleus also become the sites of oxidation attack, and then the twinned structure will be etched.5,6,19,22,24,40,43 Therefore, it is difficult to obtain twinned Pd nanocrystals with a high shape purity when the synthetic environment has both oxygen and a ligand such as a halide ion.5,6,19,22,24,43 However, synthesis of twinned Pd structures enclosed by {001} planes, such as pentagonal nanowires or rods (5-fold twin), and right bipyramids (single twin), often requires the addition of halide ions for selective adsorption.24−29 Although the yield of them could be improved by diminishing the influence of O2 or by replacing Cl− with less corrosive Br− or by tuning the concentration of I−, developing new methods for controlling the twinned structure and enhancing the purity of them, especially the purity of Pd right bipyramids, is challenging and urgently required.28,29 In this work, we demonstrate the idea that employing small organic molecules (SOMs) that can attract halide ions via induced positive charges to decrease their activity and chemical potential could greatly eliminate the oxidative etching of O2/I− (Cl−) pairs. Tuning the strength of the interactions between SOMs and I− (Cl−) ions could help us selectively synthesize 5fold twinned Pd nanowires and single twinned Pd right bipyramids in high yields and purities. We found that Pd nanowires gaining more twin boundaries are more active than Pd right bipyramids when catalyzing the reaction of 4nitrophenol reduction.

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RESULTS AND DISCUSSION

The details of the synthesis are given in the Experimental Section. Briefly, Pd nanocrystals were prepared via a hydrothermal route using a redox reaction between HCHO and Pd(II), in the presence of poly(vinyl pyrrolidone) (PVP) and NaI as the capping agents. The addition of proper SOMs, such as acetonitrile, acetone, 1,4-dioxane, 1,3,5-trioxane, ethanol, and 2-propanol, was utilized to help tune the shape and twinned structure of the final Pd nanocrystals. Figure 1a is the scanning electron microscopy (SEM) image of the product obtained when CH3CN was used, showing that under this condition the sample was composed of a large number of high-purity nanowires. The corresponding enlarged SEM image (Figure 1b) shows that the nanowires are uniform in size. The TEM image (Figure 1c) indicates that the monodisperse nanowires exhibit a solid wire structure. The measurement of 100 stochastic wires gives an average length of 490.0 nm and an average diameter of 15.4 nm. No other element except Pd can be found in the energy-dispersive X-ray spectrum (EDS) (Figure 1d) of the product (the signals of Cu and C are from the carbon-coated copper grid), revealing that the nanowires consist of elemental palladium. To clarify the microstructure of the nanowires, electronic diffraction (ED) of a typical wire was conducted. As indicated in Figure 1e, the diffraction pattern cannot be assigned to a single zone, revealing that the nanowire is not a single crystal. Actually, it can be indexed as a superposition of two basic zones of Pd, ⟨112⟩ and ⟨001⟩, showing its twinned crystalline nature.44 The reflection spots of the [−112] zone are connected by white lines and formed a rectangular lattice. Likewise, the other group of reflections is linked together with red lines to construct a square reciprocal lattice, which is from the [001] zone of face-centered cubic (fcc) Pd. Additionally, because of the overlapping of the twin domains, reflections of double diffraction that cannot be involved in the zones described above also exist. All this information supports the idea that the nanowire has a cyclic pentagonal twinned structure with five {111} end faces and five {001} side facets, elongating in the [110] direction.44 This structure usually formed when 5-fold twinned seeds anisotropically grew to a one-dimensional (1D) shape with stabilization of {001} facets.

EXPERIMENTAL SECTION

Chemicals. Sodium iodide (NaI, AR), sodium carbonate decahydrate (Na2CO3·4H2O, ACS), acetonitrile (CH3CN, >99.9%), 1,4-dioxane (C4H8O2, ACS), trioxane (>99.5%), ethanol (C2H5OH, 99.7%), and 2-propanol (99.8%) were purchased from Aladdin Co. Palladium(II) chloride (PdCl2, 99.9%) was purchased from Alfa Aesar. Acetone (>99.5%), sodium chloride (NaCl, >99.5%), poly(vinyl pyrrolidone) (PVP, K-30), and formaldehyde (HCHO, 37−40 wt % in H2O) were obtained from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Ultrapure water (18.2 MΩ) was used in all procedures. HCHO (0.1 mL) was added to 10 mL of the ultrapure water to prepare the dilute formaldehyde solution (0.37−0.40 wt %). NaCl and PdCl2 were mixed in the ultrapure water at a mole ratio of 2:1 to obtain a 0.1 M Na2PdCl4 solution. Synthesis of Pd Nanowires and Right Bipyramids. For a typical synthesis of Pd nanowires, a vial containing 0.15 g of NaI, 0.40 g of PVP, 7.2 mL of water, and 5.0 mL of CH3CN was sonicated until NaI and PVP were completely dissolved (the molar ratio of CH3CN to NaI was ∼96.22:1). After that, Na2PdCl4 (1.0 mL, 0.1 M), Na2CO3 (usually 0.80 mL, 0.1 M), and a dilute HCHO solution (1.0 mL, 0.37− 0.40 wt %) were added to the vial while its contents were being gently stirred to keep the pH value between 8.0 and 8.5. The resulting homogeneous dark red solution was transferred to a 20−21 mL Teflon-lined stainless steel autoclave. The sealed vessel was then heated at 140 °C for 4−12 h before it was cooled to room temperature (the growth had nearly reached completion after 4 h, but increasing the time to 8 and 12 h led to almost no change in size). The product was collected by centrifugation at 10000 rpm for 15 min and washed three times with ethanol. The precipitate was redispersed in 5.0 mL of ethanol and stored at room temperature for further use. When CH3CN was replaced with 5.0 mL of acetone, 5.0 mL of 1,4-dioxane, or 2.5 g of 1,3,5-trioxane, we also obtained Pd nanowires. When 7.2 2454

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Figure 2. Electronic microscope images of the products prepared when CH3CN was used after different reaction times of (a and b) 2, (c) 8, and (d) 12 h.

completion after 4 h. Unlike the results reported by Zheng,26 Ostwald ripening was not obvious here, although the heating lasted for 12 h. In our experiments, formaldehyde (HCHO) was employed as the reducing agent; its reducibility was much stronger than that of PVP. The resulting faster reduction rate could well satisfy the fast growth along ⟨110⟩ and then bring the growth process into the kinetic region to form a more pure one-dimensional structure.26 In fact, in our synthetic system, the absence of formaldehyde prevented the creation of Pd nanowires. As seen in Figure S1 of the Supporting Information, random nanoparticle aggregates formed in this case. In addition, a suitably higher pH value is also propitious for enhancing the reduction rate.15 We found that a pH value between 8.0 and 8.5 tuned by Na2CO3 was the best choice for the growth of Pd nanowires. If no Na2CO3 was added, irregularly shaped structures were obtained (Figure S2 of the Supporting Information). All these results confirm that a suitably quick reduction rate is very important for the rapid growth of nanowires. If CH3CH2OH was used to replace CH3CN, Pd right bipyramids bound by {001} facets were obtained. This type of structure was not frequently seen in previous reports,28,45−49 especially for Pd.28 Improving the yield and purity of this kind of product remains a challenge. The SEM image (Figure 3a) of the product from our synthesis demonstrates that it consists of massive polyhedra that are highly uniform in shape. The polyhedral configuration could be seen more clearly from the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image (Figure 3b). The TEM image (Figure 3c) reveals that projections of most of them have four corners, one of which is almost a right angle. This information agrees with the structure of right-triangular bipyramids exposing six {001} facets.28,45−49 Actually, the HRTEM image of this corner in Figure 3e shows obvious {200} lattice fringes perpendicular to each other, which also confirms that the product is Pd(0) combined with the EDS result (Figure 3d). The ED (Figure 3f) taken from a randomly selected right angular corner of a polyhedron presents a typical fcc reflection pattern along [001], according with the right-

Figure 1. Pd nanowires prepared with the assistance of acetonitrile after reaction for 4 h: (a and b) SEM images, (c) TEM image, (d) EDS profile, (e) typical electronic diffraction pattern of a single Pd nanowire, containing information about two groups of lattice [−112] and [001] zones, and (f) HRTEM image of a nanowire viewed along [−112] and [001] axes.

HRTEM imaging of a typical nanowire was performed to further identify the structure. As shown in Figure 1f, the obvious difference in contrast between the center and sides of the wire indicates the twinned structure. The continuous lattice fringes on two sides in the direction of elongation with an interplanar spacing of 0.231 nm could be assigned to {111} facets of Pd, which agrees well with the 5-fold twinned structure viewed from [−112] and [001] zone axes. The different contrast and increased fringe spacing in the central area of the wire are attributed to the double diffraction described above. Products prepared with different reaction times were characterized to reveal the growth process of the nanowires. It is found that reaction for 2 h produced nanorods ∼14.0 nm in diameter and 140−200 nm in length (Figure 2a,b), but the yield of Pd is quite low, indicating that the growth did not reach completion under this condition. Therefore, a longer time would allow continuous growth. After reaction for 4 h, the nanorods grew to ∼490.0 nm and the diameter changed slightly (∼15.4 nm on average) (Figure 1). It means that they would favor longitudinal growth rather than lateral growth. Actually, because of the prohibition of the binding of I− ions to {001} planes, the growth rate along ⟨110⟩ (longitudinal direction) is much faster than that along ⟨001⟩. Further increasing the time to 8 and 12 h almost could not change the size considering the error range [mean sizes of ∼500.0 nm (length) and ∼15.4 nm (thickness) for 8 h and ∼500.0 nm (length) and ∼15.0 nm (thickness) for 10 h] and morphology of the products (Figure 2c,d), demonstrating that the growth had nearly reached 2455

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obvious that the actual projections under TEM after tilts are nearly the same as the corresponding simulations. Therefore, we are sure that the polyhedra were right-triangular bipyramids. To understand the reason for the controlled formation of 5fold twinned and single twinned structures, we proposed a possible mechanism based on controlling the degree of oxidative etching of O2/halides.5,6,19,22,24,40,43 As suggested by Jin et al.,50 an electron-withdrawing group or an atom with a large electronegativity will induce neighboring hydrogen atoms into C−H bonds with some proton character. Three C−Hδ+ bonds in methyl forming a 3Hδ+ group like a claw could interact with a halide ion to form a “supergroup” by electrostatic forces, and two methyl groups of this type could grab the anion in a head-to-head fashion. A typical example is CH3CN, as described in Figure S3 of the Supporting Information. In general, the supergroups were not stable and could exchange halide ions with each other rapidly when they vibrated or collided with each other. All these processes will decrease the activity and chemical potential of halide ions. Actually, the rate of oxidation of I− by H2O2 slowed obviously with the presence of CH3CN, employing starch as the indicator (iodine starch test) (Figure S4 of the Supporting Information). Therefore, the existence of CH3CN could indeed decrease the activity of I−. The 1H nuclear magnetic resonance (1H NMR) spectra of the mixtures of CH3CN and NaI were recorded to further reveal the interactions between them. As shown in Figure S5 of the Supporting Information, after the addition of equimolar NaI, δC−H of CH3CN shifted to a lower field obviously, indicating that there are interactions between I− and hydrogen atoms in -CH3. This kind of force is similar to that of hydrogen bonds that could also lead to a larger chemical shift. In our synthetic media, Cl−, I−, and O2 coexist, resulting in an oxidative etching environment for destroying twinned Pd seeds.5,6,19,22,24,43 When CH3CN was introduced into the system, free halide ions (Cl− and I−) in solution would be tied down by CH3CN molecules with -CH3δ+ groups. Then their decreased activity could reduce the degree of oxidative etching of O2/halide. Accordingly, the 5-fold twinned seeds could avoid corrosion and grow fast to form nanowires. Furthermore, the wires are very stable in the synthetic system and could maintain their shape even after being heated for 12 h because of the protection of CH3CN, which is different from the results of Zheng.26 For CH3CH2OH, α-H and β-H are also positive because of the induction of electronegative oxygen, but α-H is much more positive than β-H. Therefore, we can deduce that methyl groups (β-H) in CH3CH2OH are less able to bind halide ions than those (α-H) in CH3CN. When CH3CH2OH was used instead of CH3CN, the activity of Cl− and I− increased, leading to weaker protection toward twinned seeds. Then the single twinned crystal nucleus formed and developed into right bipyramids. Previous reports always saved twinned seeds by eliminating the influence of O2 to stop or slow oxidative etching. The emphasis of our experiments is employing proper SOMs to influence halide ions to decrease the level of oxidation toward the twinned seeds. If SOMs were not used, only aggregates of particles formed (Figure S6 of the Supporting Information), confirming that the addition of SOMs is necessary in our synthetic media. Several other SOMs were utilized for the synthesis to confirm the rationality of the proposed mechanism. Besides acetonitrile and ethanol, acetone, 1,4-dioxane, 1,3,5-trioxane, and 2-propanol, which may have three positive neighboring C− Hδ+ bonds, were selected, and the charges of hydrogen atoms in

Figure 3. Pd right bipyramids prepared with the assistance of ethanol after reaction for 8 h: (a) SEM image, (b) HAADF-STEM image, (c) TEM image, (d) EDS profile, (e) HRTEM image of a frequently seen corner, showing {100} facets orthogonal to each other, and (f) SAED pattern of half of a randomly selected bipyramid along the [001] direction.

triangular bipyramid geometry. Size measurements of 100 particles gave the average length of the short edge (∼43.5 nm). To identify this bipyramidal shape more directly, TEM imaging of a typical particle with different tilts was performed and corresponding simulations were also conducted on a standard right-triangular bipyramid model using POV-Ray. Figure 4 shows a combination of TEM images and relative simulations for convenient comparison. During the simulation, the model was first set to a conformation that was the same as the selected polyhedron (0°), and then it was manipulated with rotations equal to those performed on the real polyhedron. It is

Figure 4. TEM images of a typical Pd right bipyramid after various tilts and corresponding simulations from a right-triangular bipyramid model using POV-Ray. 2456

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propanol resulted in Pd right bipyramids with an edge length of 35.3 nm (Figure 6). Moreover, there are also a few 5-fold twinned nanorods having approximate dimensions of 100.0 nm

× 27.0 nm in the sample prepared with acetone. Although 1,4dioxane and 1,3,5-trioxane have less positive hydrogen atoms, three axial C−Hδ+ bonds in three separated -CH2- groups can construct a potential well that could also trap halide ions effectively.50 Therefore, they were also able to promote the formation of nanowires (mean sizes of 1017.0 nm × 17.5 nm and 471.7 nm × 18.1 nm, respectively) (Figure 5c−f). All these results show that the proposed mechanism is rational to a certain extent. Further experiments examining the mechanism are ongoing in our group. Figure S7 of the Supporting Information shows the UV−vis absorption spectra of the typical nanowires and right bipyramids. Both kinds of Pd nanocrystals display an absorption band located around 270 nm. The difference between them is that bipyramids have another broad absorption peak with its maximum at ∼350 nm, which was also seen in the absorption spectrum of faceted particles.33 The as-synthesized 5-fold twinned nanowires and single twinned right bipyramids of Pd are ideal objects for investigating the influence of a twinned microstructure on catalytic activity. The catalytic reactions were conducted for the reduction of 4-NP by NaBH4. The absorption of 4-NP located at 400 nm was recorded after designed time intervals to determine the remaining concentration of the reagent according to the Lambert−Beer law. Figure 7a presents the temporal evolution of the concentration of 4-NP, when Pd nanowires (from CH 3 CN) and bipyramids (from CH3CH2OH) were selected as typical examples for the catalysts. Both show excellent catalytic activities. 4-NP was reduced ∼90% just after 2 and 4 min, promoted by nanowires and bipyramids, respectively. The concentration of NaBH4 was considered to be constant during the reaction because of its great excess. Then the kinetic fitting was conducted with respect to 4-NP only. The linear plots of ln(Ct/C0) versus time t (Figure 7b) indicate that the reactions were controlled by pseudo-first-order kinetics. Then the rate constants were calculated to be 1.173 and 0.589 min−1 for nanowires and bipyramids, respectively. They were normalized by the corresponding calculated surface areas of the two catalysts (26.0 and 17.2 m2/g, respectively) for comparison of their catalytic performance (0.635 and 0.481 min −1 cm −2 , respectively), as shown in Figure 7c. Obviously, Pd nanowires are ∼1.32 times as active as bipyramids toward this reaction. Considering that both of them are confined by {001} facets, the better catalytic performance of nanowires probably arose from the presence of more twin boundaries. Actually, defects and disordered atoms in twin boundaries not only are susceptible to oxidative etching but also may become active catalytic sites.6,40

Figure 6. (a) SEM and (b) TEM images of Pd right bipyramids obtained when 2-propanol was used.

CONCLUSIONS In conclusion, a strategy that employs small organic molecules (acetonitrile, acetone, 1,4-dioxane, and 1,3,5-trioxane; ethanol and 2-propanol) for the selective synthesis of 5-fold twinned nanowires and single twinned right bipyramids of Pd was developed. The yield and purity were quite improved. The tentative mechanism is focused on the fact that molecules having different strengths of interactions with I− (Cl−) would have different abilities to decrease the degree of oxidative etching of the O2/halide pair and then allow the control of twinned nanostructures of Pd. Because there are more twin boundaries in nanowires, they are more active than bipyramids in catalyzing the reduction of 4-nitrophenol, though they all mainly expose {001} facets. The results of this work provide a

methyl groups (methylene for 1,4-dioxane and 1,3,5-trioxane) were calculated using three methods. As listed in Table S1 of the Supporting Information, compared to CH3CH3, hydrogen atoms in methyl groups of acetone and acetonitrile have more positive charges, while those in ethanol and 2-propanol are just a little more positive. As we expected, the assistance of acetone indeed led to the formation of Pd nanowires ∼590 nm in length and ∼13.5 nm in diameter (Figure 5a,b), and the use of 2-

Figure 5. SEM and TEM images of the nanowires synthesized when other small organic molecules were employed: (a and b) acetone, (c and d) 1,4-dioxane, and (e and f) 1,3,5-trioxane.

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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 NSFC (21101119 and 21173159), the NSFC for Distinguished Young Scholars (51025207), the ZJSTD Key Innovative Team (2012R1001410), the Open Fund of State Key Laboratory of Chemical Resource Engineering in Beijing University of Chemical Technology (CRE-2012-C-101), and the Xinmiao talent project of Zhejiang Province (2012R424054).

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Figure 7. Plots of (a) Ct/C0, (b) ln Ct/C0, and (c) [ln(Ct/C0)]/S vs reaction time for the reduction of 4-NP catalyzed by Pd nanowires and right bipyramids prepared with the assistance of CH3CN and CH3CH2OH, respectively. S in panel c represents the calculated surface area of each catalyst.

new idea for tuning the degree of etching of O2/halides for controlling the twinned structure of noble metals in a hydrophilic system. In addition, this paper also advocates a method for improving catalytic performance with regard to twinned structures.

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ASSOCIATED CONTENT

S Supporting Information *

Methods of theoretical calculation, Figures S1−S6, Table S1, including additional SEM images, 1H NMR spectra, an as-called iodine starch test, and charge calculation results. This material is available free of charge via the Internet at http://pubs.acs.org. 2458

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dx.doi.org/10.1021/cm4042204 | Chem. Mater. 2014, 26, 2453−2459