Facile Synthesis and Growth Mechanism of Flowerlike Ni−Fe Alloy

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Facile Synthesis and Growth Mechanism of Flowerlike Ni-Fe Alloy Nanostructures Lijun Liu,†,‡ Jianguo Guan,*,† Weidong Shi,† Zhigang Sun,† and Jiashou Zhao‡ State Key Laboratory of AdVanced Technology for Materials Synthesis and Processing, Wuhan UniVersity of Technology, Wuhan 430070, People’s Republic of China, and Department of Chemical Engineering, Wuhan Textile UniVersity, Wuhan 430073, People’s Republic of China ReceiVed: May 9, 2010; ReVised Manuscript ReceiVed: July 2, 2010

Flowerlike Ni-Fe alloy nanostructures composed of nanorods have been synthesized via a facile hydrothermal approach at relatively low temperature (100 °C) independent of surfactants or external magnetic field. The concentrations of NaOH and FeCl3 play a crucial role in determining the morphology of the Ni-Fe alloy via adjusting the reaction rate. The excessive amount of NaOH favors the formation of alloyed phase without segregation. Such flowerlike architectures follow a stepwise growth mechanism with initial formation of nanocores aggregated from nuclei and a subsequent site-specific anisotropic growth of nanorods along their easy magnetic axis under kinetic control. The Ni-Fe flowers showed enhancement of their ferromagnetic properties, which may be attributed to the anisotropic shape and the incorporation of Fe compared to pure Ni. Our work may shed light on the designed fabrication of complex 3D architectures of other alloyed materials. 1. Introduction Hierarchical nanostructures assembled by low-dimensional building blocks including nanoparticles,1,2 nanorods,3,4 and nanoplates5,6 often exhibit significant morphology and/or sizedependent properties.7-11 As an important functional material, Ni1-xFex alloy shows extraordinary electrical, catalytic, and magnetic properties and has the potential technological applications in catalysis, sensors, electromagnetic shielding and absorbing materials, and so forth.12-14 Great efforts have been focused on the fabrication of nickel-iron alloyed micro-/nanoparticles by various physical and chemical methods.15-22 Among these methods, the solution-phase co-reduction route usually has advantages of the mildness, simplicity, low-cost, and large-scale production and has been adopted to successfully prepare some low-dimensional Ni-Fe alloy nanostructures including nanospheres19 and polycrystalline nanorods.20 However, the fabrication of 3D hierarchical Ni-Fe alloy nanostructures via a solution-phase chemical reduction approach still remains a challenge.22 This is possibly related to the remarkable difference of standard electrode potentials between Ni2+/Ni and Fe3+/Fe pairs, and the strong magnetic interaction between magnetic building blocks of Ni-Fe alloy. As a result, 1D structures rather than 3D hierarchical architectures are easily obtained because of the minimization of the magnetic anisotropic energy along their magnetic easy axis.23-25 Recently, through the so-called stepwise growth strategy, ZnO nanomaterials with complex 3D morphologies have been synthesized.26,27 It is believed that the remarkably different growth kinetics in each step results in hierarchical growth of 3D micro-/nanostructures. Moreover, with use of suitable precipitating or chelating agents, the reduction potential of Mn+/M pairs can be readily adjusted because of the decrease of concentrations of free Mn+ ions, which gives an opportunity to achieve similar reduction potentials of various Mn+/M pairs. * To whom correspondence should be addressed. Phone: +86-2787218832. Fax: +86-27-87879468. E-mail: [email protected]. † Wuhan University of Technology. ‡ Wuhan Textile University.

Herein, we report a surfactant-free, one-pot hydrothermal synthetic approach of highly uniform, flowerlike Ni-Fe alloy by directly reducing Ni(II) and Fe(III) salts with hydrazine hydrate in alkaline conditions. With use of a strategy of generating solid M(OH)n, the reduction potentials of Ni2+/Ni and Fe3+/Fe pairs are adjusted to be a similar value. A stepwise growth mechanism is rationally proposed, including an initial formation of nanocores via nuclei aggregation and a subsequent site-specific anisotropic growth of nanorods under kinetic control. Such a flowerlike Ni-Fe alloy composed of nanorods affords a model system for fundamental investigations and promising applications in electronic, magnetic, and sensing nanodevices. 2. Experimental Section All chemicals used in this work were of analytical grade and were used as received without further purification. In a typical procedure, a suspension of 24 mL was first prepared by dissolving NiCl2 · 6H2O (50 mM), FeCl3 · 6H2O (5 mM), and NaOH (0.2 M) in distilled water. Subsequently, 1.0 mL of 85% hydrazine hydrate was added to the solution as a reducing agent. The mixture was stirred vigorously and then was transferred into a Teflon cup in a stainless steel-lined autoclave. The autoclave was maintained at 100 °C for 10 h and then was cooled down to room temperature. A black fluffy solid product was deposited on the bottom of the Teflon cup, indicating the formation of nickel-iron alloys. The final product was collected by a magnet and rinsed with distilled water and ethanol several times to remove any alkaline salts, and then dried in a vacuum oven at 40 °C for 12 h. The X-ray diffraction (XRD) patterns of the samples were recorded with a Rigaku D/Max-2000 diffractometer equipped with a Cu KR radiation source (λ ) 0.15418 nm). The scanning range was from 10° to 90° and the scanning interval was 0.02°/ 2θ. Morphologies of the samples were studied by a Hitachi S-4800 field emission scanning electron microscope (FESEM). The element composition was characterized by a Horiba EX250 X-ray energy-dispersive spectrometer (EDS) associated with FESEM. Transmission electron microscopy (TEM) and selected

10.1021/jp104212v  2010 American Chemical Society Published on Web 07/22/2010

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Figure 1. (a) XRD pattern and (b) EDS spectrum of the as-synthesized flowerlike Ni-Fe alloy. The inset in (a) is the enlarged (111) peak.

area electron diffraction (SAED) were performed on an FEI Tecnai G2 electron microscope with an accelerating voltage of 200 kV. For TEM characterization, the products were ultrasonically dispersed in ethanol, and then the resulting suspension was dropped on a Cu grid coated with carbon film. The hysteresis loops were conducted by using a Model-4HF vibrating sample magnetometer at room temperature with a maximum magnetic field of 10 kOe. For magnetization measurements, the powder was pressed strongly and fixed in a small cylindrical plastic box. 3. Results and Discussion The typical XRD pattern of the product is shown in Figure 1a. Three distinctive diffraction peaks at 2θ of 44.40°, 51.74°, and 76.24° are indexed as the face-centered cubic (fcc) γ phase nickel-iron alloy. No impurity peak is found in the product. It is noted that the (111) peak of the as-synthesized Ni-Fe alloy shifts to a lower angle (ca. 0.10°) relative to the standard pattern of the pure nickel (JCPDS 04-0850). The lattice constant, a, calculated from the (111) peak is 3.530 Å, which is slightly larger than that of fcc Ni (a ) 3.523 Å, JCPDS 04-0850). This implies that Fe atoms are incorporated into the fcc Ni lattice, leading to the enlargement of the interplanar spacing as the radius of Fe atoms (1.26 Å) is larger than that of Ni atoms (1.24 Å).20 The strong intensity of the XRD pattern suggests the high crystallinity of the product obtained even at such a low temperature of 100 °C. Figure 1b shows the EDS analyses of the product. Obviously, both Fe and Ni peaks appear, and from the peak intensity the atomic percentage of Ni is measured to be 91.3% of the sum of Fe and Ni atoms, close to the theoretical value of 90.9% calculated by the content of the precursor. This proves that Ni(II) and Fe(III) salts are almost reduced to zerovalent metals. The occurrence of Al peak at about 1.5 keV

Figure 2. (a) SEM image of the flowerlike Ni-Fe alloy nanostructures. (b) TEM image and (c) SAED pattern of a single nanorod that constitutes the flowerlike nanostructure as a subunit.

is due to the aluminum foil substrate, which supports the sample during the test. From Figure 2a, it can be seen that the as-synthesized products consist of relatively uniform flowerlike architectures with diameters of ∼1 µm. Such flowerlike structure is composed of several rods with diameters of 100-200 nm in the middle section and lengths of 300-800 nm. Each rod seems to grow from the same center. This indicates that the flowerlike Ni-Fe alloy nanostructures are synthesized successfully by the surfactant-free hydrothermal reduction method. Interestingly, the flowerlike structure cannot be destroyed into discrete rods even under a long period of time of ultrasonication, indicating that the complex architectures are actually integrated and not made up of loosely aggregated nanorods through magnetic dipole interactions. Further observation from the TEM image (Supporting Information, Figure S1) clearly indicates that the flowerlike patterns are built up with nanorods, consistent with the SEM observations. Figure 2c is the SAED pattern obtained by aligning the electron beam perpendicular to the single nanorod shown in Figure 2b. It confirms that the nanorod is a single crystal and the preferential growth is along the direction of [111]. Generally, it is difficult to prepare alloy through wet chemical co-reduction routes because of their remarkably different electrode potentials between Mn+/M (M ) metals) pairs.28-30 It is known that the electrode potentials of Mn+/M (φ(Mn+/M)) change with the concentration of free Mn+ ions. With use of the strategy of complexation or deposition of Mn+ ions, φ(Mn+/ M) can be tailored effectively. In our protocol, when NaOH solution is added dropwise, the yellow-green solution containing

Flowerlike Ni-Fe Alloy Nanostructures

Figure 3. (a) XRD patterns of the samples prepared at various [NaOH]. SEM images of the samples obtained at [NaOH] of (b) 0.15 M and (c) 0.3 M.

Fe3+ and Ni2+ ions immediately turns into a muddy suspension, indicating the formation of solid hydroxides M(OH)n (M ) Fe, Ni). The free Fe3+ and Ni2+ ions sharply decrease based on the dissolution-deposition equilibrium. Because of the excessive NaOH, the reduction potential of Fe3+/Fe redox pair (-0.733 V) is close to that of Ni2+/Ni redox pair (-0.702 V). This can be calculated using the Nernst equation based on the solubility product constants (Ksp) of solid hydroxides, as shown in Illustration S1 (Supporting Information). Thus, Fe3+ and Ni2+ species can be co-reduced to metallic Ni and Fe atoms followed by generating alloyed Ni-Fe nuclei, which is a thermodynamically favorable process. To confirm the role of NaOH, we have used XRD and SEM to characterize the samples obtained at various NaOH concentrations ([NaOH]). In the absence of NaOH, the obtained product is the mixture of Ni and γ-Fe2O3 (Figure 3a). We speculate that the γ-Fe2O3 comes from the oxidation of metallic Fe during the XRD test because of the strong oxidation tendency of metallic Fe in nanometer scale. When the excessive amount of NaOH is introduced, the obtained samples are of Ni-Fe alloy phase with fcc crystallinity (Figure 3a). It can be further evidenced by Figure S2, which shows that the (111) peaks of the products obtained at [NaOH] g 0.15 M shift to a lower 2θ degree, compared with that of the pure nickel. Thus, it is

J. Phys. Chem. C, Vol. 114, No. 32, 2010 13567 reasonable to conclude that the excessive NaOH is essential to co-reduce the metallic salts and facilitates the formation of alloyed phase without segregation. Apart from controlling the phase, NaOH also plays a vital role in tailoring the morphologies of Ni-Fe alloy. In this work, the growth rate decreases with increasing [NaOH], as the time to finish the co-reduction process is observed to prolong at a higher [NaOH]. The possible reason is the decrease of free metal ions according to dissolution equilibrium of initially formed solid hydroxides, in spite of a slightly enhanced reducing capacity of hydrazine hydrate at this time. Figure 3b shows that, at [NaOH] ) 0.15 M, quasi-spheres with a diameter of ∼500 nm and a rough surface are obtained. When [NaOH] is increased to 0.2 M, perfect rod-based flowers appear (Figure 2). When [NaOH] is further increased to 0.3 M, besides a minor amount of irregular nanoparticles, most of the products are 3D flowerlike nanostructures composed of smooth nanoplates with a thickness of ∼50 nm (Figure 3c). It can be seen from Figure 3a that the intensity ratio I{111}/I{200} of the Ni-Fe alloy nanostructures shown in Figure 3c is much higher than the conventional value (3.54 vs 2.38). This implies that the basal planes of nanoplates should be the {111} facets. Similar results have also been reported for platelike fcc metals such as silver,31 nickel,32 copper,33 and gold34 plates, possibly because these {111} facets possess the lowest surface energy. In this protocol, FeCl3 not only is a precursor of alloy but also plays an important role in tailoring the reaction rate and morphology. It is found that the time to finish the co-reduction reaction is prolonged with increasing concentration of FeCl3 ([FeCl3]) in the system. In the absence of FeCl3, the reaction is completed within 2 h and the obtained samples are irregular spherical particles of ca. 400 nm in size (Figure 4a). When [FeCl3] is 2 mM, flowery particles composed of rods with a diameter of ∼200 nm are obtained after 6 h (Figure 4b). The morphologies evolve from thermodynamically favored spheres to kinetically favored anisotropic flowers when [FeCl3] rises from 0 to 5 mM. It is known that the generation rate of metal decreases when higher electrode potential pairs, e.g., Fe3+/Fe2+, are introduced into the metal reduction process, as the highvalence metallic ions have the ability to oxidize the zero-valence metal into metallic ions.35,36 For example, platinum anisotropic nanostructures were synthesized by the manipulation of reduction kinetics via the introduction of Fe3+/Fe2+ pairs.36 Accordingly, a higher [FeCl3] means a lower generation rate of Ni-Fe alloy, which favors the growth under kinetic control and a final product with a highly anisotropic structure.37 The obvious evolution indicates that when [FeCl3] is controlled in the solution, Ni-Fe alloys with well-defined morphologies and dimensions can be kinetically realized. To illustrate the morphology evolution of the flowerlike Ni-Fe alloys, we’ve used SEM to characterize the products obtained at various hydrothermal reaction stages of the typical synthesis (Figure 5). At the reaction time (t) of 3 h, the obtained intermediates are spherical nanoparticles with a diameter of ca. 100 nm (Figure 5a). The intermediates obtained at t ) 4 h are undeveloped flowerlike particles with a size of 200-300 nm (Figure 5b). It seems that several nanorods grow out of the surface of the spherical nanocore. As t proceeds to 6 h, more nanorods with different lengths and diameters appear (Figure 5c). At t ) 10 h, the small nanorods disappear and flowerlike Ni-Fe alloyed particles composed of uniform nanorods are obtained (Figure 2). The obvious stepwise growth mechanism could be observed based on the time-dependent experiments.

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Figure 4. SEM images of the products obtained at various [FeCl3] of (a) 0 and (b) 2 mM.

The solid hydroxides M(OH)n initially form when excessive NaOH solution is added to the mixture solution of NiCl2 and FeCl3. As the value of φ(Fe3+/Fe) is close to that of φ(Ni2+/ Ni) in this case, the Fe3+ and Ni2+ ions released from their solid hydroxides are simultaneously reduced to metallic atoms, which spontaneously form Ni-Fe alloyed nuclei. It should be noted that the concentration of OH- ions keeps almost constant at the initial stage because OH- ions released from the metal hydroxides are the actual demand for the reduction Ni2+ and Fe3+ ions. The relevant chemical reactions can be summarized as eqs 1-5:

Ni(OH)2 T Ni2+ + 2OH-

(1)

Fe(OH)3 T Fe3+ + 3OH-

(2)

2Ni2+ + N2H4 + 4OH- f 2Ni + N2 + 4H2O

(3)

4Fe3+ + 3N2H4 + 12OH- f 4Fe + 3N2 + 12H2O

(4) (1 - x)Ni + xFe f Ni1-xFex

(x ) 0.087)

(5)

Because of the minimization of the surface free energy and the strong magnetic dipole-dipole interaction, spherical primary

Figure 5. SEM images showing the morphology evolution of the intermediates collected after different hydrothermal reaction times of (a) 3, (b) 4, and (c) 6 h.

nanoparticles (Figure 5a) form via random aggregation of the newly formed nuclei. These primary nanospheres have many protuberances on their rough surfaces. The subsequent growth is thermodynamically favorable on such geometric curvatures which have higher chemical potentials than geometric plane surfaces. It is known that N2H4 · H2O is a Lewis base. The following dissociation equilibrium (eq 6) exists in its aqueous solution:

N2H4 · H2O T N2H5+ + OH-

(6)

As the reaction goes on, the depletion of N2H4 · H2O (eqs 3 and 4) leads to a decrease in the concentration of OH- ions, which results in an increase in free Fe3+ concentration based on dissolution equilibrium of Fe(OH)3 (eq 2). In this protocol, we speculate that the Fe3+ ions may retard the generation of Ni-Fe alloy by oxidizing the Ni(0) back to Ni2+ through the following reaction:

Flowerlike Ni-Fe Alloy Nanostructures

Fe3+ + Ni f Fe2+ + Ni2+

(EQ ) 1.034 V)

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(7)

The relatively elevated [Fe3+], weak reduction capability of N2H4 · H2O at this time and relatively strong thermodynamic tendency (EQ ) 1.034 V) facilitate the occurrence of eq 7, which effectively slows down the generation rate of Ni-Fe alloy. Such a low reduction rate leads to a low supersaturation, supplying enough time for growth units to grow along the lowest energy orientation.38 As for magnetic Ni-Fe alloy with fcc crystalline, the preferential growth direction follows the route of the minimum magnetic anisotropic energy or surface free energy. The former can achieve the minimum value when magnetic particles grow along the magnetic easy axis.38 The latter has a lower value while the exposed faces consist of more {111} facets because the sequence of surface energy for fcc structure is confirmed as γ{111} < γ{100} < γ{110}.39 A faster growth leads to a similar growth rate of various facets and isotropic morphology, e.g., spheres, is usually obtained.11 At a low growth rate, an anisotropic growth along the magnetic easy axis occurs, driven by the minimization of magnetic anisotropic energy. Further lowering of the growth rate to an extreme extent, the difference of growth rates of various facets becomes more remarkable, which facilitates the growth of high-energy facets driven by the surface free energy minimization principle.37 In this protocol, after primary nanocores are initially formed, the subsequent growth is subjected to a low rate because of the depletions of reactant and more pronounced retarding effects by Fe3+ (eq 7). The minimization of magnetic anisotropic energy prevails over that of surface free energy and dominates the anisotropic growth of the subunits. It is accepted that Ni-Fe alloy with fcc crystallographic structure has a magnetic easy axis of [111] direction.40 The nanorods should grow along the [111] direction to acquire a low magnetic anisotropic energy in the presence of a relatively slow reaction rate. Therefore, many projecting nanorods grow out of the surface of primary nanocores along [111] preferential direction (Figure 5b) and further grow into Ni-Fe alloy nanorods with various diameters and lengths (Figure 5c). At the last stage, some nanorods continue to grow as the aging time increases, while a number of smaller nanorods disappear, possibly because of “Ostwald Ripening”.41 The particles finally develop into a beautiful flowerlike assembly with several nanorods sharing a nanocore (Figure 2). From the above discussion, we believe the kinetic parameters determine the growth rate of various facets and finally modify the shape and dimensionality of the subunits. It should be noted that the nanoplates constructing the flowerlike nanostructures shown in Figure 3c have different growth mechanisms, compared with the formation of nanorods discussed above. At 0.3 M NaOH, the growth process is subjected to an extremely low rate. The minimization of surface free energy dominates the growth rates of various facets. The growth rate of {111} facets is much lower than that of {110} because of their lowest surface free energy.39 The subunits take a preferential growth, possibly along the [110] or [100] direction and expand to a nanoplate with {111} as the basal planes.42 From the above discussion, the growth mechanism for flowerlike Ni-Fe alloy nanostructures composed of nanorods is illustrated in Figure 6. We measured the M-H loops of the as-synthesized flowerlike and quasi-sphere Ni-Fe alloys at room temperature (Figure 7). The specific saturation magnetization (Ms) of the flowerlike Ni-Fe alloy is 60.02 emu/g, which is slightly higher than that of bulk Ni (55 emu/g).43 It is known that Ms is proportional to

Figure 6. Schematic illustration for the stepwise growth mechanism of the flowerlike Ni-Fe alloy nanostructures.

Figure 7. Magnetic hysteresis loops of the Ni-Fe alloy obtained at [NaOH] of 0.15 M (solid dots) and 0.20 M (hollow dots). The inset shows the enlargement of M-H loop at low magnetic field.

the number of Bohr magnetons. The number of Bohr magnetons for iron is slightly higher than that of nickel.14 Thus, the increase in Ms of our Ni-Fe alloy is caused by the increase in the number of Bohr magnetons because some Ni atoms in the crystal lattice have been replaced by Fe atoms. Compared with the quasisphere particles, the flowerlike Ni-Fe alloy has a lower saturation magnetization because of its higher specific surface. It is known that high specific surface can increase the surface spin disorder and antiferromagnetic oxidation, which lead to the reduction of ferromagnetism.40 In the low-field region, a hysteresis loop exists (inset in Figure 7), indicating ferromagnetic behavior of the as-obtained samples. The coercivity Hc for the flowerlike and quasi-spherical Ni-Fe alloy is determined as 118.6 and 83.7 Oe, respectively, both showing great enhancement compared with that of bulk nickel (∼0.7 Oe44 at room temperature). Despite the same composition, the flowershaped Ni-Fe alloy has a higher coercivity than the quasispherical shaped one. Generally, the magnetization behaviors of magnetic materials are highly dependent on the shape and crystalline anisotropy.45 For the flowerlike Ni-Fe alloy, the nanorods as subunits have a remarkable anisotropic shape. This is a major contribution to the enhanced coercivity. Moreover, compared with the highly symmetrical fcc structure of nickel, the symmetry of Ni-Fe alloy incorporated by 8.7 at. % Fe atoms slightly decreases because the size of Fe atoms is different from that of Ni atoms, leading to an increase in the crystalline anisotropy. From the above discussion, we believe the anisotropic shape of the subunits and the slight crystalline anisotropy account for the high coercivity of flowerlike Ni-Fe alloy. 4. Conclusions In summary, flowerlike fcc Ni-Fe alloy (8.7 at. % Fe) composed of nanorods is prepared by a facile surfactants-free hydrothermal process without any induced magnetic field. As built units, the alloyed nanorods grow preferentially along the [111] direction, which is the magnetic easy axis of fcc Ni-Fe

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alloy. The excessive NaOH thermodynamically favors the formation Ni-Fe alloyed phase. The FeCl3 in the system can kinetically control the growth rate of the Ni-Fe alloy by oxidizing the Ni(0) to Ni(II), which plays a key role in the formation of flowerlike microstructures. The stepwise growth mechanism is proposed based on the time-dependent experiments. Compared with the bulk nickel, the flowerlike Ni-Fe alloy exhibits enhanced ferromagnetic behavior. The higher saturation magnetization comes from the incorporation of 8.7 at. % Fe and the increased coercivity ascribes to the anisotropic shape and slight crystalline anisotropy during the alloyed process. Acknowledgment. We gratefully acknowledge the support of this work by the National High Technology Research and Development Program of China (No. 2006AA03Z461) and the National Defense Fundamental Scientific Research Program (No. A1420080185). Supporting Information Available: The TEM image of the as-obtained flowerlike nanostructures, enlarged (111) peaks of the samples obtained at various [NaOH], and illustration of calculating electrode potentials of Ni2+/Ni and Fe3+/Fe pairs at 0.2 M NaOH. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Baek, J. Y.; Ha, H. W.; Kim, I. Y.; Hwang, S. J. J. Phys. Chem. C 2009, 113, 17392. (2) Tong, G. X.; Guan, J. G.; Xiao, Z. D.; Mou, F. Z.; Wang, W.; Yan, G. Q. Chem. Mater. 2008, 20, 3535. (3) Nikoobakht, B.; Wang, Z. L.; El-Sayed, M. A. J. Phys. Chem. B 2000, 104, 8635. (4) Wang, D. H.; Choi, D. W.; Yang, Z. G.; Viswanathan, V. V.; Nie, Z. M.; Wang, C. M.; Song, Y. J.; Zhang, J. G.; Liu, J. Chem. Mater. 2008, 20, 3435. (5) Mou, F. Z.; Guan, J. G.; Sun, Z. G.; Fan, X. A.; Tong, G. X. J. Solid State Chem. 2010, 183, 736. (6) Zhu, L. P.; Zhang, W. D.; Xiao, H. M.; Yang, Y.; Fu, S. Y. J. Phys. Chem. C 2008, 112, 10073. (7) Kanaras, A. G.; Sonnichsen, C.; Liu, H.; Alivisatos, A. P. Nano Lett. 2005, 5, 2164. (8) El-Sayed, M. A. Acc. Chem. Res. 2004, 37, 326. (9) Ciszek, J. W.; Huang, L.; Wang, Y.; Mirkin, C. A. Small 2008, 4, 206. (10) Yuan, Z. Y.; Zhou, W. Z.; Su, B. L. Chem. Commun. 2002, 1202. (11) Xia, Y.; Xiong, Y.; Lim, B.; Skrabalak, S. E. Angew. Chem., Int. Ed. 2009, 48, 60. (12) Ferrando, R.; Jellinek, J.; Johnston, R. L. Chem. ReV. 2008, 108, 845. (13) Liao, Q. L.; Tannenbaum, R.; Wang, Z. L. J. Phys. Chem. B 2006, 110, 14262.

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